It's important to be careful when posting links to spam in chat.
Most spam on Stack Exchange is link spam, a malicious (or at least unwanted) attempt to get people to click a link.
A link in chat to a link spam post is not, generally speaking, itself spam. But most of them time when a user posts a link to a Stack Exchange post in chat, it is oneboxed--expanded to a box that shows a preview of its text. If that preview contains a live link to an external spam site, then there is actual spam in chat.
The presence of a live link to a spam site in chat--remember, this is different from just a link to a spam post on AU--achieves the same sort of goal for the spammer as their original post. In particular, it survives the deletion of the original spam post, and may continue garnering clicks for the indefinite future. Furthermore, chat logs are indexed by search engines. If you create oneboxed posts that display live spam links, even if your goal was to fight spam, you are propagating spam. Such posts absolutely should be flagged. But they are best not flagged as spam.
Since a user who does this is not usually spamming intentionally, rather than flagging a oneboxed chat post that immortalized an external spam link as spam, the appropriate action is to flag it with a custom moderator flag. This is usually actioned relatively quickly since moderators from any Stack Exchange site can handle custom moderator flags in any SE chat room (even if it's associated with a different site).
As Oli says, the solution for people using chat to coordinate anti-spam efforts is to include other text in your post besides the link, so that it is not oneboxed, when posting links to spam posts.
This is a known issue.
I'm posting this separate answer to raise attention to how this has happened before, that it could be mitigated by changes to the Stack Exchange chat system, and that having your oneboxed spam flagged as spam in chat does not mean anyone is out to get you:
Flagging oneboxed spam in chat that has clickable external links is the wrong thing for someone to have done, because flagging for mod attention is more appropriate and avoids unnecessary punitiveness.
With that said, posting a live link to an external spam site in chat is a more seriously wrong thing to do: the main difference between it and actual spamming is intent. (On the other hand, maybe you did not do that, and I am not saying you did; I didn't see the chat post in question.)
And neither wrong thing is all that serious. In a case where neither a user who inadvertently oneboxes link spam payloads, nor the other user who see and flags their chat posts as spam, is actually attempting to abuse the system, then in the end no major harm has been done.
Finally, note that any user who has immortalized links to external spam sites in chat oneboxes can fix the problem by going back and flagging the posts for moderator attention. (Oneboxed spam that doesn't contain clickable links, on the other hand, it almost certainly not worth bothering moderators to remove--such posts are even occasionally considered of lasting value.)